The use of fluorescent dyes as markers or tags for liquid and solid materials is well known. A typical application is the tagging of liquids such as hydrocarbon fuels in order to identify the liquid at a subsequent point in the supply chain. This may be done for operational reasons, e.g. to assist in distinguishing one grade of fuel from another, or for other reasons, in particular to ensure fuel quality, deter and detect adulteration and to provide a means to check that the correct tax has been paid. Apart from fuels, other products, such as vegetable oils may be marked to identify the product produced at a particular source, or certified to a particular standard.
A problem with the method of detecting fluorescent compounds used as markers arises when the material which is marked interferes with the fluorescence of the marker by absorbing the excitation or emitted light, by exhibiting its own background fluorescence, or by changing the fluorescent characteristics of the marker. This is a particular problem in the marking of coloured liquids, such as petroleum derived products, with fluorescent dyes. Hydrocarbon based liquids, such as fuels, exhibit a broad fluorescent emission. The fluorescent background tends to add to any fluorescent signal of the dye but the inherent absorbance of the liquid diminishes the fluorescence of the dye. The marking of such fuels, especially gasoline and diesel, is an important use of marker compounds and the ability to detect single or multiple marker compounds with a high degree of certainty is critical to the use of such markers in such valuable and widespread products. The problem has been addressed in many ways, most of which involve the separation of the marker compound from the liquid by means of extraction into a polar liquid or onto a solid absorbent. For example, U.S. Pat. No. 5,358,873 describes and claims a method of detecting gasoline adulteration by tagging with a rhodamine dye and then shaking a small sample of the suspected fuel in a vial containing a small quantity of unbonded flash chromatography-grade silica. The presence of the rhodamine marker dye in the suspect sample colours the silica red. U.S. Pat. No. 4,659,676 describes a fluorescently labeled complex hydrophobic fluid produced by dissolving therein a porphyrin. The fluorescently labeled complex hydrophobic fluid is identified by observation of the characteristic fluorescence upon irradiation. For identification purposes the porphyrin may be first extracted into an acidic aqueous solution for determination of fluorescence. U.S. Pat. No. 2,392,620 describes the use of umbelliferone or a derivative as a fluorescent marker for petroleum with detection by determination of the characteristic fluorescence after extraction into an aqueous alkaline solution. In U.S. Pat. No. 4,735,631, fuels are marked with certain substituted anthraquinones which are subsequently detected in a marked sample of fuel by extraction into an immiscible alkaline reagent.